External combustion engines, such as, for example, Stirling cycle engines, have traditionally used tube heater heads to achieve high power. FIG. 1 is a cross-sectional view of an expansion cylinder and tube heater head of an illustrative Stirling cycle engine. A typical configuration of a tube heater head 108, as shown in FIG. 1, uses a cage of U-shaped heater tubes 118 surrounding a combustion chamber 110. An expansion cylinder 102 contains a working fluid, such as, for example, helium. The working fluid is displaced by the expansion piston 104 and driven through the heater tubes 118. A burner 116 combusts a combination of fuel and air to produce hot combustion gases that are used to heat the working fluid through the heater tubes 118 by conduction. The heater tubes 118 connect a regenerator 106 with the expansion cylinder 102. The regenerator 106 may be a matrix of material having a large ratio of surface to area volume which serves to absorb heat from the working fluid or to heat the working fluid during the cycles of the engine. Heater tubes 118 provide a high surface area and a high heat transfer coefficient for the flow of the combustion gases past the heater tubes 118. However, several problems may occur with prior art tube heater head designs such as inefficient heat transfer, localized overheating of the heater tubes and cracked tubes.
As mentioned above, one type of external combustion engine is a Stirling cycle engine. Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression. The Stirling cycle refrigerator is also the mechanical realization of a thermodynamic cycle that approximates the ideal Stirling thermodynamic cycle. Additional background regarding aspects of Stirling cycle machines and improvements thereto are discussed in Hargreaves, The Phillips Stirling Engine (Elsevier, Amsterdam, 1991).
The principle of operation of a Stirling engine is readily described with reference to FIGS. 2a-2e, wherein identical numerals are used to identify the same or similar parts. Many mechanical layouts of Stirling cycle machines are known in the art, and the particular Stirling engine designated by numeral 200 is shown merely for illustrative purposes. In FIGS. 2a to 2d, piston 202 and displacer 206 move in phased reciprocating motion within cylinders 210 that, in some embodiments of the Stirling engine, may be a single cylinder. A working fluid contained within cylinders 200 is constrained by seals from escaping around piston 202 and displacer 206. The working fluid is chosen for its thermodynamic properties, as discussed in the description below, and is typically helium at a pressure of several atmospheres. The position of displacer 206 governs whether the working fluid is in contact with hot interface 208 or cold interface 212, corresponding, respectively, to the interfaces at which heat is supplied to and extracted from the working fluid. The supply and extraction of heat is discussed in further detail below. The volume of working fluid governed by the position of the piston 202 is referred to as compression space 214.
During the first phase of the engine cycle, the starting condition of which is depicted in FIG. 2a, piston 202 compresses the fluid in compression space 214. The compression occurs at a substantially constant temperature because heat is extracted from the fluid to the ambient environment. The condition of engine 200 after compression is depicted in FIG. 2b. During the second phase of the cycle, displacer 206 moves in the direction of cold interface 212, with the working fluid displaced from the region cold interface 212 to the region of hot interface 208. The phase may be referred to as the transfer phase. At the end of the transfer phase, the fluid is at a higher pressure since the working fluid has been heated at a constant volume. The increased pressure is depicted symbolically in FIG. 2c by the reading of pressure gauge 204.
During the third phase (the expansion stroke) of the engine cycle, the volume of compression space 214 increases as heat is drawn in from outside engine 200, thereby converting heat to work. In practice, heat is provided to the fluid by means of a heater head 108 (shown in FIG. 1) which is discussed in greater detail in the description below. At the end of the expansion phase, compression space 214 is full of cold fluid, as depicted in FIG. 2d. During the fourth phase of the engine cycle, fluid is transferred from the region of hot interface 208 to the region of cold interface 212 by motion of displacer 206 in the opposing sense. At the end of this second transfer phase, the fluid fills compression space 214 and cold interface 212, as depicted in FIG. 2a, and is ready for a repetition of the compression phase. The Stirling cycle is depicted in a P-V (pressure-volume) diagram shown in FIG. 2e. 
The principle of operation of a Stirling cycle refrigerator can also be described with reference to FIG. 2a-2e, wherein identical numerals are used to identify the same or similar parts. The differences between the engine described above and a Stirling machine employed as a refrigerator are that compression volume 214 is typically in thermal communication with ambient temperature and the expansion volume is connected to an external cooling load (not shown). Refrigerator operation requires net work input.
Stirling cycle engines have not generally been used in practical applications due to several daunting challenges to their development. These involve practical considerations such as efficiency and lifetime. The instant invention addresses these considerations.